Internal combustion engines must meet ever stricter emissions and efficiency standards demanded by consumers and government regulatory agencies. Accordingly, automotive manufacturers and suppliers expend great effort and capital in researching and developing technology to improve the operation of the internal combustion engine. Turbochargers are one area of engine development that is of particular interest.
A turbocharger 2, such as that shown in FIG. 1, uses exhaust gas energy, which would normally be wasted, to drive a turbine wheel 10. The turbine wheel 10 is mounted to a shaft 12 that in turn drives a compressor wheel 20. The turbine wheel 10 converts the heat and kinetic energy of the exhaust into rotational power that drives the compressor wheel 20. The objective of a turbocharger is to improve the engine's volumetric efficiency by increasing the density of the air entering the engine. The compressor draws in ambient air and compresses it into the intake manifold and ultimately the cylinders. Thus, a greater mass of air enters the cylinders on each intake stroke.
When a conventional turbocharger is sized to provide maximum power output for a particular engine, the turbocharger's low-load and transient response performance is generally less than optimal. A turbocharger's compressor performance is dependent on the compressor speed. In order for the compressor to rotate fast enough to provide significant compression, or boost, to the engine, there must be a corresponding increase in exhaust gas flow. However, there is a time delay while the exhaust gases build up and the inertia of the turbine and compressor wheel assembly is overcome. This time delay between the engine's demand for boost and the actual increase in manifold pressure is often referred to as turbo lag.
To help overcome the problems of turbo lag and low-load performance, electrically-assisted turbochargers have been developed. Electrically-assisted turbochargers include an electric motor that is operative to supplement the rotational power derived from the exhaust during low-load and transient conditions. Typically, the motor is connected to the same shaft that carries the turbine and compressor wheels. In some cases, the motor's rotor magnets are carried directly on the shaft, while the stator is contained within the turbocharger's bearing housing.
Referring again to FIGS. 1 and 2, the typical turbocharger bearing housing 60 is cast as a single unitary piece using, for example, a sand cast process that employs various sand-based cores to produce features in the casting. Certain features of the bearing housing, such as air gaps and oil passages 81-83 are difficult, if not impossible, to cast into a single piece housing because the sand-based cores have thin sections and are therefore too delicate to withstand the metal pouring process. Furthermore, various cross passages 81-83 that cannot be cast into the single piece housing must be machined into the housing through the journal bearing bore (77, 78). Accordingly, insertion of the drill used to machine the cross passages is geometrically impeded by the solid structure of the bearing housing. As a result, the bearing and cross-passage configuration is constrained. The drill path for passages 81 and 82 are depicted as 95 and 94, respectively, in FIG. 2.
As is well known in the art, the rotating assembly 14 that comprises shaft 12, turbine wheel 10, and compressor wheel 20 should be dynamically balanced in order to achieve the necessary rotational speed without self-destructing at high speeds, often in excess of 200,000 RPM. The rotating assembly 14 should be balanced as an assembled unit, i.e., the compressor wheel 20 and various small parts assembled to the turbine wheel 10 and shaft 12. If the compressor wheel 20 is removed and re-installed, for example, the rotating assembly 14 must be re-balanced, as the alignment of the compressor wheel 20 to the shaft 12, the clamp load of the nut on the compressor wheel, etc., change the balance of the rotating assembly 14. Therefore, with the typical one-piece cylindrical bearing housing 60, this balancing cannot occur until after the rotating assembly 14 is assembled to the bearing housing 60. The design of the rotating assembly 14 is constrained in that it must allow for assembly of the components within the unitary housing 60. This includes bearings 49 and seals 52 as well as the rotating assembly's components.
In the case of an electrically assisted turbocharger, the rotating assembly includes motor components. For example, the rotor is attached to the shaft. Also, the stator surrounds the motor and must be inserted into the bearing bore. It should be appreciated that the assembly of all of these components occurs out of view within the housing.
Although the traditional single piece bearing housing has served the industry well, it does have disadvantages, as explained above. Accordingly, there is still a need for a bearing housing that allows for flexibility in rotor assembly and bearing design. There is a further need for a bearing housing design that facilitates manufacturing, balancing, and testing complex rotor assemblies associated with electrically assisted turbochargers.